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Two Rivers Gravel Pit Mine <br /> Riverside Berm Failure Analysis <br /> and Flood Control Mitigation Plan <br /> 7ech"olooicess January 22,2020 <br /> WinDAM C estimates the amount of time for a dam (riverside berm)to head cut/erode. If the pit <br /> fills in less time than the head cutting erosion time, then head cutting/erosion ceases. That is <br /> because if the water surface elevation driving head cutting/erosion meets that in the pit, then <br /> there is no head differential for the head cutting/erosion process. <br /> In addition, WinDAM C estimates head cutting/erosion length from the pit rim with respect to <br /> time. Thus, the head cutting length at the time that the pit fills to the rim is known and can be <br /> used for sizing riverside berm width. For instance, say the pit fills in 4 hours and head cutting <br /> and erosion ceases at that time. And, during that 4 hours the head cut length is 50 feet. Then, a <br /> riverside berm width of 50 ft should be adequate. However for conservatism, a safety factor(say, <br /> 2 X) can be added. <br /> Two Rivers pits fill times are based on a combination of groundwater ingress with no dewatering <br /> (dewatering pumps stopped when flood warning received), right floodplain (South Platte River <br /> side) 100-yr flood inflow, and left floodplain (Big Thompson River side) 100-yr inflow. That <br /> information is presented in the Appendix. <br /> Calculations of pit fill times involves a reservoir(pit)routing procedure. That is, inflow equals <br /> outflow minus change in pit storage. Because there is no outflow in the pits, then inflow equals <br /> change in storage. An elevation-capacity table was developed and the flood hydrograph <br /> described above in subsection 3.2.3, Hydrograph Development, converted to volumes and <br /> cumulated. The cumulated volumes then compared to the elevation-storage table and inflow <br /> hydrograph times, and pit elevations vs fill times determined. This process is presented in the <br /> Appendix. <br /> 3.3.2 Flood Warning Time <br /> Should a large flood be imminent, appropriate flood threats will be issued by the Colorado Water <br /> Conservation Board (CWCB). And once flood threats are issued, Varra Companies, Inc will stop <br /> dewatering in active pits allowing for groundwater ingress which will result in higher pit water <br /> surface elevation, and decreased head cutting/erosion. <br /> Should a flood be imminent, a flood threat warning will be issued by CWCB and Varra <br /> Companies, Inc. stop dewatering thereby allowing pits to begin filling. Thus, an extra measure of <br /> confidence can be added to the results presented below because those results are based allowing <br /> groundwater ingress and higher pit water surface elevations. <br /> The Colorado Flood Threat Portal (http://www.coloradofloodthreat.com/)issued by the (CWCB) <br /> includes a daily statewide flood threat bulletin and map, 7-15 day flood threat outlook, and <br /> statewide 24-hr precipitation map. That will be monitored by Varra Companies, Inc on an as- <br /> needed basis. <br /> 3.4 If head cut/erosion length is less than riverside berm width at the time that head cut elevation <br /> equals pit water surface elevation, then head cutting/erosion ceases and will not capture the river <br /> Refer to IV, Results. <br /> pg. 14 <br />